The invention relates to a process for nucleus-chlorination of aromatic hydrocarbons in the presence of Friedel-Crafts catalysts and in the presence of co-catalysts in the liquid phase.
The reaction of aromatic hydrocarbons, such as toluene, in the liquid phase with gaseous chlorine to give nucleus-substituted chlorine derivatives, such as monochlorotoluene, is known from the literature (Ullmanns Encyklopadie der technischen Chemie (Ullmann's Encyclopedia of Industrial Chemistry), 4th edition, Volume 9, page 499 et seq.). The chlorination is in general carried out in the presence of Friedel-Crafts catalysts, such as iron(III) chloride, antimony chlorides or aluminium chloride. The resulting chlorination product is usually a mixture of monochlorinated and polychlorinated position isomer compounds. Thus, for example, a mixture of monochlorotoluenes and dichlorotoluenes is obtained in the chlorination of toluene catalyzed with iron(III) chloride. The main product in the monochlorotoluene fraction is o-chlorotoluene. This fraction also contains p-chlorotoluene and a small amount of m-chlorotoluene.
Since p-chloroalkylbenzenes in particular, such as p-chlorotoluene, are useful intermediate products, there has in the past been no lack of attempts to steer the chlorination such that the ratio of o- to p-chloroalkylbenzenes is reduced, that is to say attempts have been made to discover conditions which promote the formation of p-chloroalkylbenzenes.
It is known from U.S. Pat. No. 3,226,447 that an o/p ratio of 1.2 can be obtained by adding sulphur compounds with divalent sulphur to the Friedel-Crafts catalyst in the chlorination of, for example, toluene. A disadvantage of this process is the fact that this favourable ratio is achieved only when antimony salts are used as the Friedel-Crafts catalysts. It is furthermore a disadvantage that, according to Example 16, the amounts of catalyst components used are very high, that is to say 1% by weight for each of the two components. Another disadvantage is that the o/p ratio is still significantly greater than 1, that is to say more o-compound than p-compound is formed.
The chlorination of toluene with Friedel-Crafts catalysts or compounds which form Friedel-Crafts catalysts under the reaction conditions with the addition of sulphur or sulphur compounds, such as sulphur halides, is described in DE-OS (German Published Specification) No. 1,543,020 and U.S. Pat. No. 4,031,144. A ratio of o/p for the chlorination of toluene of 1.03-1.10 is thereby obtained (see the example table of U.S. Pat. No. 4,031,144). The amounts used are, for example according to Example 9 of DE-OS (German Published Specification ) No. 1,543,020, 0.05% by weight of S.sub.2 Cl.sub.2 and 0.10% by weight of FeCl.sub.3. A disadvantage of this process is that the o/p ratio is still unsatisfactorily high.
The chlorination of toluene with Friedel-Crafts catalysts with the addition of thianthrenes or substituted thianthrenes is known from U.S. Pat. No. 4,031,147, U.S. Pat. No. 4,069,263, U.S. Pat. No. 4,069,264 and U.S. Pat. No. 4,250,122. The favourable o/p ratios which can be achieved for the chlorination of, for example, toluene are about 0.7. The disadvantage of this process, however, is that this favourable ratio is achieved either only by using antimony salts as the Friedel-Crafts catalyst or, in the case of the use of iron salts as the catalysts, only at very low reaction temperatures of about 0.degree. C. (see Example 14 of U.S. Pat. No. 4,250,122 and Examples 2-4 of U.S. Pat. No. 4,069,263). Both circumstances are industrially decidedly unfavourable. When antimony salts are used, the co-catalytic effect of the thianthrenes is greatly hindered by the traces of iron which are to be avoided industrially only at exceptional expense (in this context, see U.S. Pat. No. 4,024,198). In addition, the reaction is so highly exothermic that removal of the heat at about 0.degree. C. is very expensive (cooling with brine). A further disadvantage is that--as taught in DE-OS (German Published Specification ) No. 3,023,437--the thianthrenes are already destroyed by the ubiquitous traces of water under the customary reaction conditions and thus lose their activity.
The chlorination of toluene in the presence of Lewis acids and phenoxathiines as the catalyst system is known from the Patent Specifications U.S. Pat. No. 4,289,916, European Pat. No. 0,063,394 and European Pat. No. 0,173,222. The o/p ratio which can be achieved is in one case 0.6 (see Example 1 of European Pat. No. 0,173,222). A disadvantage here is, however, the industrially extremely unfavourable use of antimony chloride and the high amount of 0.29% by weight of co-catalyst used (see Example 1 of European Pat. No. 0,173,222). If iron(III) chloride is used instead of antimony chloride, an o/p ratio of 0.68 results (see Example 3 of European Pat. No. 0,173,222), but this in turn is obtained only at the industrially extremely unfavourable low reaction temperature of 5.degree. C. At the industrially advantageous reaction temperature of 50.degree. C., the o/p ratio with the phenoxathiine derivative claimed in European Pat. No. 0,173,222 in the presence of iron(III) chloride is only 0.88, as experiments carried out by ourselves show (see Example 40). In the other two patent publications (U.S. Pat. No. 4,289,916 and European Pat. No. 0,063,384), the best examples are described with amounts of in each case 0.05% by weight of iron(III) chloride and 0.05% by weight of a phenoxathiine derivative used at a reaction temperature of 35.degree. C. and a most favourable o/p ratio of about 0.8. If the industrially less favourable antimony chlorides are used here instead of iron(III) chloride, the o/p ratio can be reduced down to 0.65 at a reaction temperature of 20.degree. C. (see Example No. 16 of European Pat. No. 0,063,384). The disadvantage is, however, that phenoxathiines are also destroyed in the presence of traces of water.
The chlorination of toluene in the presence of Friedel-Crafts catalysts and N-substituted phenothiazines is known from the patent specification European Pat. No. 0,126,669. According to the most favourable example (see Example No. 1 of European Pat. No. 0,126,669), an o/p ratio of 0.84 results at a reaction temperature of 30.degree. C. using amounts of 0.011% by weight of iron(III) chloride and 0.028% by weight of co-catalyst. The o/p isomer ratio in this process is still unfavourably high.
The chlorination of, for example, toluene in the presence of certain zeolites is known from European Pat. No. 0,112,722 and European Pat. No. 0,154,236. The o/p ratio is about 0.3 when, for example, halogenocarboxylic acid halides are added as moderators. The considerable amounts of 5% by weight of zeolite and 1% by weight of halogenocarboxylic acid halides used are a disadvantage of this process. A further considerable disadvantage is that very large amounts (up to 8% by weight) of benzyl chlorides are obtained in the mixtures obtainable, as our own experiments have shown. The formation of benzyl chlorides interferes with the subsequent customary distillative working up to a quite exceptional degree.